1. Field of the Invention
The present invention relates to a high-temperature fuel cell separator. More specifically, the present invention relates to a high-temperature fuel cell separator which is capable of maximizing performance of a stack through efficient utilization of individual oxygen and nitrogen components supplied from an oxygen/nitrogen separator of a fuel cell system and which is also capable of improving reliability of a stack through inhibition of high-temperature region formation.
2. Description of Related Art
Generally, a fuel cell is a power generation system which employs hydrogen as a fuel and atmospheric oxygen as an oxidant and generates electricity with formation of water through oxidation-reduction reactions of hydrogen and oxygen.
Unlike other conventional power generation systems, fuel cells are attracting a great deal of interest as a next-generation alternative energy source, as they entail substantially no environmental pollution and noise and exhibit high-power generation efficiency.
In particular, a molten carbonate fuel cell (MCFC) or solid oxide fuel cell (SOFC), which is a high-temperature type fuel cell, is a system that produces electricity at high temperatures of 500° C. or higher. This type of a power generation system requires no use of a noble metal catalyst (such as platinum) for oxidation of hydrogen and reduction of oxygen, which therefore readily allows use of poisonous gas such as carbon monoxide, so it is possible to utilize coal gas as a fuel. In addition, it is also advantageous to exploit high-temperature waste heat, thus providing high efficiency of the system.
A unit cell of the fuel cell system is comprised of fuel and air electrodes where electrochemical reactions take place, a separator for forming flow paths of fuel and oxidant gases, a collector plate for collecting electric charges, and an electrolyte/support for ionic conduction. As used herein, the term “stack” refers to a multi-layered structure of unit cells.
The separator provides electrical connection between the unit cells, and simultaneously serves to offer a flow path of fuel gas to the fuel electrode and a flow path of oxidant gas to the air electrode.
In the fuel cell system as configured above, some of the energy of a fuel converts into electrical energy, whereas most of the remainder thereof converts into heat which will contribute to formation of a high-temperature region inside the stack.
The resulting high-temperature region will have adverse effects on individual components of a fuel cell, such as electrodes, electrolyte and separator.
In other words, a life span of the fuel cell stack may be significantly shortened due to various factors such as high temperature-induced structural changes in porous electrodes, increased corrosiveness and deformability of metal separator, and consequently leakage of fuel gas.
As a conventional art to inhibit the occurrence of such a high-temperature region, U.S. Pat. No. 7,097,929 discloses a technology of reducing gas flow-induced pressure by shortening a length of a flow path for oxidant gas consisting mainly of air, which makes it possible to supply large amounts of oxidant gas, thus inhibiting the occurrence of a high-temperature region. U.S. Pat. Nos. 5,175,062 and 5,660,941 disclose a technology to inhibit the occurrence of a high-temperature region, wherein heat necessary for reforming reactions is supplied from heat generation of the stack.
Conventionally, a separator based on an excess supply of oxidant gas has a fuel electrode flow path for migration of hydrogen fuel gas and carbon dioxide and an air electrode flow path for migration of air and carbon dioxide, wherein the fuel electrode portion and the air electrode portion are sequentially disposed.
Elevation of pressure takes place due to resistance to migration of excess gas in a given flow path, so a molten carbonate fuel cell among high-temperature fuel cells is configured to have a structure where fuel gas and oxidant gas are isolated from each other by way of an electrolyte impregnated in porous ceramic, for example, in the form of a wet seal.
Therefore, when an excess of the oxidant is supplied to inhibit the occurrence of the high-temperature region, this leads to inordinate elevation of pressure in the flow path, which consequently results in destruction of the wet seal, thus causing leakage of fuel gas and significant deterioration in a life span of the fuel cell stack. These problems have been solved up to now with a pressurized system, but there still remain disadvantageous limitations such as high complexity of the system and difficulty of system operation.
On the other hand, a separator intended to inhibit the occurrence of a high-temperature region of the stack taking advantage of an internal reforming reaction is configured to have a structure including a reforming chamber inside the separator, such that heat produced from electrode reactions can be used for heat absorption of the reforming reaction.
However, since such a separator has a structure with inclusion of a reforming catalyst, poison of the reforming catalyst may limit a life span of the stack. Additionally, when it is desired to use coal gas, there is a disadvantage associated with a need for re-conversion of produced hydrogen into methane prior to use thereof.
As another conventional art, there is a method of using oxygen as oxidant gas, by means of an oxygen/nitrogen separator. This technology is intended to improve performance of the stack by elevating oxygen partial pressure of the air electrode. Improvements of the stack performance may result in reduction of internal heat generation, but it is inevitable that only limited inhibitory effects on formation of a high-temperature region of the stack are obtained.